1. Field of the Invention
The present invention relates to a position control system capable of fast positioning a read/write device to a particular track position of a recording medium such as a disk having therein a tracking pattern.
2. Description of the Related Art
A magnetic recording/reproducing system for magnetically reading/writing an information signal has been used in various technical fields because reading/writing an information signal from/to the magnetic recording medium is easily performed. As the demands for higher density recording/reproducing become greater, the track pitch has been made narrower. With the narrower track pitch, it is difficult for a magnetic head to trace the track. Various tracking methods have been proposed to solve the above problem. For instance, widely adopted in the art is to record signals or patterns in a magnetic disk for tracking control.
One example of such tracking control patterns is optical patterns which are recorded beforehand in a magnetic floppy disk as the tracking control information. By reading such optical patterns, the magnetic head can reliably be tracked at a particular narrow track under the tracking control operation.
Such optical patterns for tracking purposes have been formed heretofore on the surface of or within the magnetic recording medium layer of a floppy disk. Various types of examples of a magnetic head capable of reading the tracking information, writing/reading the data information signal to/from a magnetic disk are disclosed, e.g., in Japanese Patent Publication No. 58-41577, Japanese Patent Laid-open Publications Nos. 56-148770, 59-8172.
FIGS. 3(a) to 3(c) are plan, front, and side views, respectively showing the main part of a magnetic recording/reproducing apparatus which uses a magnetic floppy disk with optical patterns to read/write information while detecting the optical patterns.
Referring to FIGS. 3(a) to 3(c), the apparatus comprises a motor 1, rotary encoder 2, a cassette 3 for housing therein a magnetic floppy disk D, a read/write device 4 which is mounted on a carriage 5 on which components for reading the tracking optical patterns are also mounted, such as, a pulley 6, a steel belt 7 and a disk drive motor 9.
As the motor 1 rotates, the carriage 5 moves in the direction indicated by an arrow 8 because one end of the steel belt 7 is wound around the pulley 6 which is fixedly coupled to the motor 1 shaft and the other end of the steel belt 7 is fixed at the carriage 5. The motor 1, pulley 6, steel belt 7 and carriage constitute an actuator for the read/write device 4.
The carriage 5 and hence read/write device 4 is caused by a controller to move in the direction indicated by the arrow 8, i.e., in the radial direction of a magnetic floppy disk during a seek mode operation. The controller measures a distance to a target track based on a given target address and an address read from the track, and controls to move the read/write device at a speed corresponding to the measured distance (this control is conducted within the range indicated as speed control in FIG. 11).
In a position control mode (tracking control mode), the control is conducted within the range indicated as position control in FIG. 11. This tracking control is carried out in accordance with a signal output from an optical pickup device for picking up the tracking optical pattern, to thus allow reliable tracking control. The optical pickup device is mounted facing the part of the above-described components mounted on the carriage 5. An example of the pickup element for tracking an optical pattern is disclosed, e.g., in U.S. Pat. application Ser. No. 259,706 filed on Oct. 19, 1988, now U.S. Pat. No. 4,987,505.
FIG. 10 is a fundamental block diagram showing the servo system used during the seek mode operation corresponding to the speed control shown in FIG. 11. FIG. 9 is a fundamental block diagram showing the servo system used during the position control or tracking control mode operation corresponding to the position control shown in FIG. 11.
Referring to FIGS. 9 and 10, both the servo systems are constructed of a digital signal processor DSP, e.g., MN 1901 type manufactured by Matsushita Electric Industrial Co., Ltd., and a carriage drive section the DSA. Digital signals from the DSA are processed in the DSP and the processed result is supplied to the DSA for control of the read/write device. A multi-peak band-pass filter BAF shown in the DSP of FIG. 9 is used as a band amplification filter. As the main functions of both the servo systems, pulse transfer functions of elementary constituent blocks and computational equations performed in the DSP are shown in Table 1. The structural arrangements of DSAs in FIGS. 9 and 10 are the same so that a fraction thereof is omitted in FIG. 10.
Expansion and contraction caused by temperature variation of polyester base material of a magnetic floppy disk is anisotropic so that a track run-out frequency of an eccentric disk has a frequency component f and 2f, f being an inverse of a revolution period Td of the disk. The band amplification filter BAF is provided so as to make a residual off-track sufficiently small over the range of two frequencies f and 2f.
TABLE 1 ______________________________________ Pulse transfer function of BAF w(z)/u(z) = (a1+a2z.sup.-1 +a3z.sup.-2)/(1-a4z.sup.-1 -a5z.sup.-2) .times. (b1+b2z.sup.-1 +b3z.sup.-2)/(1-b4z.sup.-1 -b5z.sup.-2) = {w'(z)/u(z)}{w(z)/w'(z)} Pulse transfer function of speed feedback v(z)/x(z) = c1(1-z.sup.-1) Pulse transfer function of speed detection v'(z)/x(z) = d1(1-z.sup.-1) Computational equations by BAF in DSP w'n = a1un+a2u(n-1)+a3u(n-2)+a4w"(n-1)+ a5w'(n-2) wn = blw'n+b2w'(n-1)+b3w'(n-2)+b4w(n-1)+ b5w(n-2) Computational equation by speed feedback in DSP vn = c1{xn-x(n-1)} Computational equation by speed detection (for seek) in DSP v'n = d1{xn-x(n-1)} ______________________________________
The internal variations un, w'n, wn and vn shown in Table 1 change with a rotational phase of the disk during the steady state. During the steady state, the absolute speed of the track run-out can be detected based on the value vn because the residual off-track is very small during the steady state. Also, the track run-out speed can be detected based on the value wn because the value (wn-vn) is very small relative to the value vn.
The analysis (including linear approximation) of the block diagram of FIG. 9 for the position control (tracking control) mode operation corresponding to the position control shown in FIG. 11 is shown in FIG. 12, whereas the similar analysis of the block diagram of FIG. 10 for the seek mode operation corresponding to the speed control shown in FIG. 11 is shown in FIG. 14. FIGS. 13(a) and 13(b) are Bode diagrams for BAF.
In the representations used in FIGS. 12 and 14, J denotes an inertia (g.cm.sec.sup.2) of a motor and carriage, kt a torque constant (g.cm/A), k3 a radius (micron/rad) of a pulley, k1 a forward gain, k2 a speed feedback gain, T a sampling period (sec), .tau.1 and .tau.2 a delay time (sec) caused by computation, and k4 a gain.
With the conventional position control system for a read/write device as described above with FIGS. 9 to 14, the following problems arise. Namely, it takes a long time to change from the seek mode to the position control mode if the center of a track moves swingingly about the read/write device, or an overshoot of the read/write device may occur in the width direction of a track at the time of switching from the seek mode to the position control mode (refer to FIG. 15). The problem of such long access time and unstable settling has been long desired to be solved.